The present invention relates in general to driver circuits and, in particular, to modulation circuits for laser diodes.
Laser diodes are presently employed in optical radar systems, for instance, which determine the range or distance between the radar and an object of interest. Modulator circuits selectively turn such diodes on and off. One prior art modulator circuit, for instance, employes a silicon controlled rectifier (SCR) connected with the laser diode and a capacitor. Controller trigger signals are applied to the gate of the SCR to render the SCR conductive to thereby discharge the capacitor through the laser diode to provide a transmitted light pulse. Other prior art circuits sometimes use a delay line in place of the capacitor. The delay line operates as a pulse forming network. Also, bipolar transistors or MOSFETs have been substituted for the SCR in some prior art configurations.
Other prior art approaches utilize either a mercury reed relay or an avalanche transistor as the switching element. Avalanche transistors have questionable reliability and uncontrollable discharge characteristics. The mercury reed switches are disadvantageous because they have low duty cycle and cannot be turned off by the control circuits.
The foregoing prior art circuits are useful for many applications wherein the rise time, fall time and/or pulse duration of the emitted light pulse are not critical. "Rise time" can be defined as the time required for the leading edge of a pulse to rise from 10% to 90% of its final value. It is a measure of the steepness of the wave front. Alternatively, "fall time" can be defined as the length of time for a pulse to decrease from 90% to 10% of its maximum amplitude. Pulse duration can also be called pulse length or pulse width. It is the time interval between points at which an instantaneous value on the leading and trailing edges bears a specified relationship to the peak pulse amplitude, for instance.
Typically the fastest light pulse rise times for prior art SCR circuits is approximately 10 nanoseconds. Such circuits are useful in optical radar systems for measuring ranges from 80 to 90 feet, for instance. The accuracy of such ranging devices is proportionate to the rise time and pulse width of the emitted pulse of light. These prior art circuits are useful for double aperture, laser diode, optical radar systems.
Alternatively, common aperture optical radars utilize one lens through which the optical pulses are transmitted and the reflected pulses are received. For these radars, it is important to precisely control the rise time, fall time and pulse duration of the transmitted pulse so that it does not interfere with the reflected pulse. High pulse repetition rates with short durations are required for measuring the range to close targets which may be within 30 feet of the radar. To obtain good distance resolution it is desirable to have pulse widths of 5 nanoseconds or less.
When the switching device of some of the above-described prior art circuits is rendered conductive, a capacitor delivers current to the laser diode. The internal series storage resistance of the capacitor and the conductors between the capacitor and the laser diode tend to limit the peak current flow. Also, the inductance provided by the conductors tends to limit the driving pulse rise time. Moreover, the total energy being delivered to the laser diode tends to change if the pulse repetition rate is increased. Furthermore, the capacitance of the capacitor tends to undesirably change with temperature change. This is because the dielectric factor of the capacitance changes with temperature, for instance. If the capacitance increases, more energy would be delivered to the laser diode thereby undesirably lengthening the pulse width with temperature increase, for instance. Thus, the prior art circuits are generally not suitable for use with close range, single aperture radar systems.